(1) Field of the Invention
This invention relates to a rubber composition comprising a nitrile group-containing highly saturated copolymer rubber and an ethylenically saturated copolymer rubber.
The nitrile group-containing highly saturated copolymer rubber in the rubber composition of the present invention is characterized as having a specific alkylthio group and exhibiting good high-rate vulcanizability, and a vulcanizate made therefrom has high mechanical strength and low compression set.
(2) Description of the Related Art
In recent years, a nitrile group-containing highly saturated copolymer rubber (a typical example of which is a hydrogenation product of an acrylonitrile-butadiene copolymer) has attracted much attention as a rubbery material having good heat resistance, oil resistance and weather resistance.. Many nitrile group-containing highly saturated copolymer rubbers have heretofore been proposed. For example, a hydrogenated acrylonitrile-conjugated diene copolymer rubber which is a hydrogenated product of an acrylonitrile-conjugated diene copolymer and has good ozone resistance and gasoline resistance is described in Japanese Unexamined Patent Publication (JP-A) No. 54-132647.
To enhance the ozone resistance and weather resistance of a nitrile group-containing highly saturated copolymer rubber, a proposal has been made wherein an ethylene-propylene rubber (hereinafter abbreviated to "EPR") is incorporated in the highly saturated copolymer rubber (JP-A-58-40332).
Both a nitrile group-containing highly saturated copolymer rubber and EPR have a low degree of unsaturation, and therefore, are difficult to vulcanize with a sulfur-containing vulcanizer and a vulcanizate thereof has a poor mechanical strength and a large compression set.
For injection molding of an acrylonitrile-conjugated diene copolymer rubber, a high fluidity and a high-rate vulcanizability, i.e., a high crosslinking efficiency at high-temperature and short-time vulcanization, are generally required. However, in general, the higher the vulcanization temperature, the lower the degree of crosslinking efficiency. Therefore injection-molded articles have poor tensile stress and impact resilience as compared with compression molded articles (for example, see Journal of Society of Rubber Industry, Japan, vol. 59, No. 4, p 214-215, 1986). This tendency is conspicuous in a hydrogenated acrylonitrile-conjugated diene copolymer rubber.
To impart a high-rate vulcanizability to a hydrogenated acrylonitrile-conjugated diene copolymer rubber, many proposals have been made, which include incorporation of vulcanization promotors such as a tellurium dithiocarbamate compound (JP-A-6-9822), a thiuram promotor plus a partial ester salt of a di- and/or tricarboxylic acid (JP-A4-264145) and a thiuram promotor Plus N-trichloromethylsulphenylbenzenesulfanilide (JP-A-2-248442). However, the high-rate vulcanizability attained by these proposals is not on a high level and in some cases physical properties of the vulcanizate are deteriorated.
Further, contamination of a mold is conspicuous in the injection molding of a composition comprised of a hydrogenated acrylonitrile-conjugated diene copolymer rubber and an ethylenically saturated copolymer rubber, wherein vulcanization is carried out at a high temperature. That is, where the molding is repeated, staining materials are gradually deposited on the mold with the result of contamination of moldings and spoiling of the surface appearance. Therefore the mold must be cleaned at regular intervals, but the cleaning is time-consuming and costly and leads to reduction in productivity.
To avoid the contamination of a mold, proposals have been made wherein talc, sodium thiosulfate, carbon wax or silicone oil is incorporated in a rubber. However, it is difficult to avoid the mold contamination to any appreciable extent where the vulcanization is effected at a high temperature and a high rate as in injection molding.